Such a method is described in the article by H. J. W. van Houtum et al. in Journal of Vacuum Science Technology, B G(G), November/December 1988, pp. 1734-1739.
Advanced techniques are used for contacting and interconnecting elementary components in modern manufacturing technology for integrated semiconductor circuits with high packing densities, in which very large numbers of semiconductor circuit elements of very small dimensions are provided on a semiconductor slice or "chip". These techniques render it possible to manufacture contacts and interconnections of a very small width but low electrical resistance in a reproducible manner.
For this purpose, a method as described in the article referred to above is mostly used, in which a conductor pattern of polycrystalline silicon is converted over at least part of its thickness into metal silicide, having a considerably lower resistance than silicon. If metal silicide interconnections (usually called "straps") are to be formed also on the field oxide, the metal present on the oxide is coated with a layer of amorphous silicon in the relevant locations.
The surface with the silicide pattern is then coated with a dielectric layer which consists of, for example, pyrolytically deposited silicon oxide obtained through decomposition of gaseous silicon compounds. This dielectric layer, which is not flat owing to the presence of level differences on the surface, is then planarized by means of known photoresist and back-etching techniques. The required contact windows are subsequently etched into the planarized dielectric layer.
Since the distance from the surface of the planarized dielectric layer to the underlying metal silicide is not the same in all spots, than above the silicon region comprising the active semiconductor zones and bounded by the field oxide, the contact windows will be of unequal depth, so that the etching time for the "deep" contact windows will be much longer than for the "shallow" ones. Since it is the object to manufacture all contact windows during the same etching step, the metal silicide in the "shallow" contact windows will be exposed to the etching agent for an unnecessarily long time. The result of this is that, even in an etching process of comparatively great selectivity, the metal silicide underneath the "shallow" contact windows will disappear completely or almost completely, which increases the contact resistance in these contact windows to an unacceptable degree.